1. Technical Field of the Invention
This invention relates to noise reduction devices in fluid flow systems and more particularly to a differential velocity device for use downstream of a valve in a fluid flow system.
2. Description of Related Art
Control valves are used in process industries to control flow of fluids, both liquids and compressible fluids. Aerodynamically generated noise is inherent in the throttling process of gases and vapors. Throttling occurs by opening or closing a selected valve in a fluid flow system.
It is generally accepted that exposure to high levels of noise can damage the hearing of individuals working near fluid flow systems. In the United States, the Occupational Safety and Health Administration (OSHA) limits noise levels of worker exposure for the purpose of hearing conservation. For example, presently noise levels are limited to 90 decibels on the A weighted scale (dBA) for eight hour exposure. Some other countries limit exposure to 85 dBA.
Since noise generation is inherent in the throttling process, many control valves require some method of noise reduction. Often globe type valves are supplied with low noise trim using cages with a multiplicity of small drilled holes.
A more cost effective solution is desirable for moderate service conditions. Specifically some form of noise reduction that can be obtained at moderate cost is desirable for rotary control valves.
A noise reduction device comprising a central section and an outer annular section is provided. The outer section is designed to reduce the velocity of fluid flow through the device and create an annular fluid flow that has a reduced velocity when compared to the core fluid flow of the system. The preferred method for reducing the velocity of the annular flow is a staged pressure reduction wherein the fluid flow passes through an upstream aperture into a pressure reduction chamber and then through an offset downstream aperture of larger cross sectional area than the upstream aperture. The core flow of the system passes through a plurality of apertures in a central section of the noise reduction device to increase the frequency of the noise in the core flow. The device creates a flow regime with an annular flow surrounding a core flow, the annular flow having a reduced velocity compared to the core flow.
The present invention is intended to provide noise reduction of 15-20 decibels over a wide range of operating conditions. The one piece device is readily machined from wrought material such as austenitic stainless steel. In spite of the drilled holes the thick sections provide an extremely high natural frequency to prevent failure due to flow induced vibration.